FIG. 6 illustrates a configuration of a transformer 5 disclosed in Japanese Laid-open Patent Publication No. 2006-013094. The transformer 5 includes a primary winding 10 configured by electrical wires and a secondary winding 11 configured by a metal plate-shaped member.
Then, the primary winding 10 and the secondary winding 11 are alternately implemented in a core inner leg 6a in order to provide a good magnetic coupling between the primary and secondary windings. As a result, there is no need for a bobbin.
Further, the secondary winding 11 is configured by a copper plate in order to deal with a high current, and is integrally provided with heat-releasing portions 22 and 23 that are pendent outside from a core 6. This permits an improvement in heat releasing in the secondary winding.
In the above-described transformer 5 illustrated in FIG. 6, leads 24 of the secondary winding 11 configured by a metal plate are directly soldered to a printed circuit board (not illustrated). In other words, the load of the transformer is supported by the leads 24 of the secondary winding 11.
When the above-described transformer 5 is driven at a high frequency, an effective thickness of a metal plate used for a secondary winding is about twice the skin depth with respect to the frequency, taking into consideration the influence of a skin effect. This makes it possible to make the secondary winding thinner. A transformer can be made smaller if a secondary winding is made thinner as described above.
FIG. 7 illustrates a relationship between a frequency of operation and a skin depth in a metal plate (copper plate). As can be seen from the characteristics of a skin effect of copper illustrated in FIG. 7, the skin depth is 0.1 [mm] when a current of 500 [kHz] flows.
If a transformer having the above-described configuration is manufactured and operated under this condition, the transformer can be made smaller by configuring a secondary winding with a copperplate such that the thickness of the secondary winding is 0.2 [mm].
However, if the thickness of the secondary winding formed of copper plate is made thin, it will become difficult to support the transformer using the secondary winding soldered to a printed circuit board due to a decrease in strength.